1. Field of the Invention
This invention relates generally to a powertrain for a hybrid electric vehicle (HEV) having an engine, an electric machine and a multiple-speed, powershift transmission. In particular, the invention pertains to using the powertrain to prevent rollback of the vehicle on an incline.
2. Description of the Prior Art
A powershift transmission is an example of a power transmission for a motor vehicle in which there is no torque-converter. A powershift transmission is a geared mechanism producing multiple gear ratios in forward drive and reverse drive and having two input clutches, which connect a power source, such as an engine or electric motor, to two transmission shafts. A powershift transmission transmits power alternately to the two shafts using synchronized clutch-to-clutch shifts.
A powershift transmission incorporates gearing arranged in a dual layshaft configuration between the transmission input and its output. One input clutch transmits torque between the input and a first layshaft associated with even-numbered gears; the other input clutch transmits torque between the transmission input and a second layshaft associated with odd-numbered gears. The transmission produces gear ratio changes by alternately engaging a first input clutch and running in a current gear, disengaging the second input clutch, preparing a power path in the transmission for operation in the target gear, disengaging the first clutch, engaging the second clutch and preparing another power path in the transmission for operation in the next gear.
Conventional vehicles with automatic transmissions have torque converters that allow the transmission to stay in gear even when the vehicle is stopped or launching on an incline. In a vehicle having a converterless transmission, such as a powershift or manual transmission, the clutches must be slipping or disengaged when the vehicle is stopped or launching on an incline to avoid vehicle stall; therefore the transmission cannot transmit torque to the wheels immediately.
When a vehicle without a torque converter transitions from being stopped on a positive incline to ascending the incline, delays in torque delivered to the wheels necessary to accelerate the vehicle can result in undesirable rollback of the vehicle. Rollback control is required when the driver depresses the accelerator pedal, called a “tip in,” and expects to ascend the hill. If the engine is to provide torque to the wheels to launch the vehicle, a delay occurs before torque is transmitted to the wheels due to delay in charging the intake manifold and engine cylinders with a combustible fuel/air mixture and delay in producing transmission input clutch torque capacity.
When a HEV stops on a positive grade, the engine is typically shut off to save fuel, but certain conditions may require the engine to continue running including charging the battery, a driver's request for front accessory drive for air conditioning, or a request for other belt driven electro-mechanical devices. Vehicle rollback of an HEV with the engine running can occur in the transition from holding the vehicle on the positive grade to accelerating the vehicle up the grade. The HEV vehicle can be held stationary on a positive incline when the driver applies the wheel brakes, or, if the brakes are released, when an electric machine provides holding torque while the engine is idling. In a HEV, the transmission can be placed in neutral and an electric machine operating as a motor may be used to hold the vehicle stationary on an incline.
There is a need in the industry for a technique that eliminates unintended rollback due to delay in torque delivery to the wheels of a vehicle when the vehicle operator tips-in to accelerate the vehicle on an uphill grade from a stationary vehicle condition while the engine is running.